This invention is directed to the field of surface mount technology and more particularly to an apparatus and method for applying adhesives and/or solder to printed circuit boards, flexible circuits, wafers and other substrates.
The surface mount technology (SMT) assembly industry requires the application of surface mount materials such as adhesives and solder onto printed circuit boards (PCBs), flexible circuits and wafers. Components are glued in place on the PCB before normally going through a wave solder process to solder the components to the PCB. One of the methods of depositing glue onto the PCB is with a glue dispensing machine. This is a sequential process where glue is dispensed through a fine orifice point by point under control of an X-Y positioning machine. (See, e.g. U.S. Pat. No. 4,584,964 to Engel). Glue and solder printing can also be done by stencil printing, which is a parallel process. Material is applied to a stencil and then forced onto a substrate through openings in the stencil. (See, e.g. U.S. Pat. No. 5,627,108 to Alibocus et al.; U.S. Pat. No. 5,740,730 to Thompson; and U.S. Pat. No. 5,873,512 to Beilock et al.).
These current techniques generally lack the flexibility to handle the variety of shapes and heights of glue and solder paste deposits which may be needed due to the diversity of components that are often used. For example, a 0603 chip component may require a glue deposit 13 mils wide by 33 mils long by 8 mils high. At the same time, a plastic leaded chip carrier (PLCC) may require a glue deposit 60 mils wide by 120 mils long by 30 mils high. In addition, current stencil printing techniques are generally ill-suited to handle preexisting components that may be as much as 50 mils high.
There is a further requirement where it is desirable to print glue on a PCB that already has clinched connector leads protruding through the PCB. In this case, the required vertical clearance for preexisting board components alongside the adhesive deposition sites poses the manufacturing dilemma of producing controllable-volume, micro-fine apertures in a substrate thick enough to clear the preexisting components.
It is also necessary in the SMT industry to deliver solder balls in a ball grid array (BGA) to a substrate, such as a BGA package or a silicon wafer with active circuits. The substrate has pads that need to have flux placed on their surface. BGA balls are placed into the flux on the pads through the use of a stencil. The tacky flux holds the balls onto the substrate. During the solder reflow process, the solder balls melt and become permanently attached to the substrate. It is desirable to bring the ball drop stencil in contact with the substrate and drop the balls into the flux deposits over the pads on the substrate. However, it is also desirable to keep the flux away from the bottom side of the stencil. Current techniques include a structure with a chemical etch relief on the bottom side of the stencil and a small hole etched all the way through the stencil. This is a very difficult process to control.
There is therefore a need in the SMT industry to provide a cost-effective, flexible and easily-controllable means for applying viscous materials, such as adhesives, conductive glue, solder paste, and solder balls, onto PCBs, flexible circuits, wafers and other substrates.
The present invention discloses an apparatus and method for depositing adhesives, conductive glues, solder paste, solder balls and other viscous surface mount materials onto printed circuit boards (PCBs), flexible circuits, wafers and other substrates. The invention can accommodate preexisting surface mount components and materials. The invention comprises a stencil with two or more layers and utilizes material reservoirs, relief areas and delivery apertures.
In one aspect of the invention, a stencil for applying surface mount materials is disclosed which comprises at least two layers. The layers comprise at least one reservoir pocket and at least one delivery aperture and may further comprise at least one relief area. The delivery aperture delivers surface mount materials from the reservoir pocket to a surface. The relief area provides clearance for preexisting components on the surface so as to allow the delivery aperture to contact the surface. The reservoir pockets, relief areas and delivery apertures can be combined within any number of layers. In a preferred embodiment, the number of layers used is two or three, and the layers are made of metal and manufactured by the processes of chemical etch, laser cut, or electroforming. The layers can be aligned through the use of registration pins and etched registration holes and attached to one another by means of an aqueous dry-mount solder mask laminate.
In another aspect, a stencil for applying surface mount materials is disclosed which comprises: an upper reservoir layer with reservoir pockets; a middle separation layer with relief areas and reservoir through pockets; and a lower contacting layer with delivery apertures and relief openings. The delivery apertures deliver measured surface mount materials from the reservoir pockets to the surface by way of the reservoir through pockets in the middle separation layer which connect to the reservoir pockets in the upper reservoir layer. The lower contacting layer has relief openings which are connected to the relief areas in the middle separation layer.
In another aspect, a stencil for applying solder balls in a desired pattern onto a substrate is disclosed which comprises: an upper layer with at least one ball drop reservoir aperture; and a lower contacting layer with at least one relief delivery aperture to provide clearance for flux on pad sites on the substrate.
In yet another aspect, a method for depositing surface mount materials onto a surface is disclosed which comprises the steps of: matching relief areas in the stencil with preexisting surface mount components on a surface; affixing the stencil to the surface; applying surface mount materials to the stencil such that the surface mount materials fill reservoir pockets in the stencil; and depositing surface mount materials onto the surface through delivery apertures on the stencil, wherein the delivery apertures draw the surface mount material from the reservoir pockets.